This invention relates to a process for purifying to a higher degree by the use of activated carbon and an organic solvent a maltose solution of a purity of 75 to 90% which is obtained by hydrolyzing starch and subsequently subjecting the resultant hydrolyzate to an ordinary method of purification.
Maltose is a disaccharide sugar composed of two glucose molecules. Its sweetness is rated at one third of that of sugar, maltose is possessed of properties similar to those of sugar and is characterized by its high anticeptic effect and high moisture retaining property.
In recent years, more people have come to favor foods that are only mildly sweet. Use of maltose as a way of decreasing sweetness and, at the same time, retaining other properties intact or having them enhanced, is gaining popularity and use of maltose is increasing. Examples of maltosecontaining products currently available in the market include malt syrup, high-maltose syrup and plain maltose. The maltose purities in these commercial products are respectively on the order of 20 to 30%, 40 to 50% and 75 to 90%. These products are used in confectioneries and other foodstuffs. In addition, maltose finds utility as a medium for microorganic cultivation and as a raw material for industries in general. Further, maltose purified to an exceptionally high purity of more than 98.5% is used in medicines, especially in solution (for injection) intended for supplementing the sugar supply of patients of diabetes mellitus.
Various methods for producing maltose have been known and the purity of the maltose obtained by these methods has been on the order of 75 to 90%. A typical example of these methods comprises heating and treating with .alpha.-amylase a starch slurry prepared by suspending starch in water for thereby liquefying the starch, hydrolyzing the liquefied starch with .beta.-amylase and .alpha.-1,6-glucosidase to afford a maltose solution, then filtering the maltose solution and subsequently subjecting the filtrate to an ordinary purifying treatment such as by use of activated carbon and an ion-exchange resin. The purity of the maltose product thus obtained is from 75 to 90%. The greater part of the impurities in this maltose solution comprises glucose and those oligosaccharides such as maltotriose and maltotetraose which are composed of three or more glucose units. For the maltose solution mentioned above to be useful as a medicine or reagent, it must be purified further.
For the purpose of this further purification of the maltose solution, there has been used a method involving the crystallization of maltose. The crystallization of maltose is accomplished by a method using an organic solvent or a method of boiling.
In the case of the method involving the use of an organic solvent, ethanol or some other suitable organic solvent is added to the maltose solution to lower the solubility of maltose and allow the maltose selectively to be crystallized out. This method, however, has a disadvantage in that such oligosaccharides as maltotriose and maltotetraose which coexist in the solution are also crystallized out at the same time. There is another disadvantage that, when this method is practised on a commercial scale, the operation proves uneconomical because the amount of the organic solvent used as described above is very large and because the crystals of maltose produced are too fine to permit efficient recovery thereof.
In the case of the method by boiling, the maltose solution is concentrated to a fixed degree of supersaturation and, in the course of boiling, seed sugar is added thereto to permit selective growth of maltose crystals. This method also has a disadvantage that the growth of maltose crystals is impeded by the oligosaccharides such as maltotriose and maltotetraose coexisting with maltose in the solution, the growth of crystals requires a long time, the formed crystals are not capable of efficient centrifugal separation and the efficiency of recovery of these crystals is extremely poor.
There has also been disclosed a process for producing maltose of high purity free from glucose and oligosaccharides composed of three or more glucose units, which process comprises treating the liquefied starch solely with .beta.-amylase for thereby hydrolyzing the glucose chain of starch molecule into maltose units gradually from the nonreducing end inwardly while allowing the remaining portion of the glucose chain posterior to the point of branching, namely .beta.-limit dextrin, to remain unhydrolyzed in the original form of a large molecule, and adding a suitable amount of an organic solvent to the resultant hydrolyzate for thereby precipitating and separating the .beta.-limit dextrin. This method, however, has a disadvantage that the yield of maltose on the basis of starch is only on the order of 30 to 40%.
An object of this invention is to provide a process for the purification of a maltose solution which is free from the various disadvantages suffered by the conventional methods for the purification of maltose, specifically a process for recovering a maltose solution of a high purity of more than 98.5% in a high yield from a maltose solution of a purity of 75 to 90% by removing from the original maltose solution those highly obstinate oligosaccharides, such as maltotriose and maltotetraose, composed of three or more glucose units.
Buy the words "maltose solution" used in the present specification is meant a solution wherein the principal component of the solids present therein is maltose and the other components thereof are glucose and oligosaccharides composed of three or more glucose units.
The words "maltose purity" are used herein to mean the pure maltose content in the solids present in a given maltose product.
The words "high-purity maltose solution" mean a solution whose maltose purity is more than 98.5%.